Generally, in an automatic transmission, a transmission control unit (TCU) compares vehicle speed appearing as rotative speed of an output driven gear to an opening degree of a throttle indexed by the accelerator pedal and selects a suitable shift stage reflecting the vehicle speed and the throttle opening degree.
Further, when the vehicle is driving at a certain shift stage, if an automatic shift control system receives a signal that the accelerator pedal is released, in order to prevent shift shock caused by a sudden close of the throttle, the automatic shift control system synchronizes the automatic transmission to a higher shift stage than the present shift stage to maintain the vehicle speed and prevent shift shock.
In a conventional automatic transmission, if the signal that the accelerator pedal is released inputs to the shift control system at a certain shift stage driving state, the shift control system compares data corresponding to the throttle opening degree and the present vehicle speed with a preset data in a program.
Thereafter, the shift control system confirms whether the data is within the range of preset data corresponding to an objective shift stage predetermined for upshifting to a higher shift stage. If the data is within such a range, the shift control system proceeds with a predetermined shift process by control of a duty ratio for synchronization of a corresponding higher shift stage.
In the above conventional automatic transmission, if the throttle opening degree is rapidly changed, as shown in FIG. 3, the shift pattern to a higher shift stage is continuous such as I stage II stage III stage or II stage III stage IV stage such that it generates multi-shift shocks and, accordingly, shift quality is reduced. Further, the multi-shift causes a reduction in the lubricant's viscosity by repetitive operation of the operative parts and reduces durability of the hydraulic lines by frequent changes of the line pressure.